The present invention relates to a photosensitive member wherein amorphous silicon is used as its photoconductive material and to a process for forming images using the photosensitive member.
For the past several years, attention has been focused on the application to photosensitive members of amorphous silicon (hereinafter referred to as "a-Si") which is produced by the glow discharge decomposition process or sputtering process. Similarly attention has been directed to amorphous silicon-germanium (hereinafter referred to as "a-Si:Ge") having improved sensitivity in the region of long wavelengths for use in forming images by a semiconductor laser. Such promising application is attributable to the fact that for use in photosensitive members, a-Si and a-Si:Ge are exceedingly superior to the conventional selenium and CdS materials in resistance to environmental pollution, heat and abrasion, photosensitive characteristics, etc.
However, a-Si or a-Si:Ge has the drawback of being low in dark resistivity and unusable as it is for the photoconductive layer serving also as a charge retaining layer. It has therefore been proposed to incorporate oxygen or nitrogen into the material to improve the dark resistivity, but this conversely results in reduced photosensitivity, hence there is a limit to the content of the additive
Accordingly it is proposed to give improved charge retentivity by forming over the photoconductive layer a light-transmitting a-Si insulation layer having oxygen or carbon incorporated therein (e.g. U.S Pat. No. 4,465,750). Nevertheless, improved chargeability requires a higher carbon concentration, which needs to be at least 70 atomic % in some cases. Overcoat layers of such high carbon concentration are difficult to make by the common glow discharge decomposition process Moreover, the overcoat layer, if obtained with a high carbon concentration, exhibits poor adhesion to the photoconductive layer (of a-Si or a-Si:Ge), possibly permitting the photosensitive member to create blank streaks in the copy images produced. Thus, there is a limitation to the improvement of chargeability by increasing the carbon content.
When the photosensitive member having an a-Si or a-Si:Ge photoconductive layer is used for forming copy images by common xerography, electrostatic latent images are formed on the surface of the photosensitive member. In other words, the electrostatic latent image is formed by the charges retained on the surface. However, since the member is low in charge retentivity as stated above, the surface charges readily disappear or decay, failing to give a satisfactory copy image. Especially it is diffucult to obtain a multiplicity of copies from a single electrostatic latent image.